Discretizers are widely used in creating air laid fibrous structures. One common type of discretizer used in air conveyed processes is the hammer mill. These devices are typically comprised of some type of system to deliver solid additives, such as fibers, for example pulp fibers, via air. An example of one system used by a discretizer to deliver solid additives is an infeed roller system through which webs of pulp pass into a housing that contains a large rotor. This rotor can contain a plurality of swinging hammers that contact the web of pulp to discretize individual pulp fibers from the web. The device may further contain a screen through which the individualized pulp fibers move for further use. The rotation of the rotor, however, imparts a strong asymmetry to the fluid, for example air, coming out of the device, for example hammer mill, that causes a non-homogeneous mixture of the pulp and air to exit the hammer mill.
Prior Art FIG. 1A illustrates an example of a discretizer that is commercially available from DanWeb. As shown in FIG. 1A, a discretizer 10 comprises a housing 12 that contains a rotor 14 having a plurality of swinging hammers 16 and a screen 18 through which individualized solid additives, for example pulp fibers, may travel through. As the rotor 14 rotates in the direction of the arrow (counter-clockwise), a web of pulp 19 is introduced into the discretizer 10 such that the swinging hammers 16 individualize pulp fibers from the web of pulp 19. The rotation of the rotor 14 imparts a strong asymmetry to the air coming out of the discretizer's intermediate discharge outlet 20A as shown in the general air flow arrows and shown in the air flow map 22. As shown by the air flow map 22, a large, but not complete portion of the air flowing out of the discretizer's intermediate discharge outlet 20A is flowing in the general direction of A. However, there is a component of the air flow that creates one or more zones of separated flow B. These one or more zones of separated flow B result in the air flow map 22 having a component that has a directional flow into the discretizer 10 not out of the discretizer's intermediate discharge outlet 20A.
In this particular example, the housing 12 of the discretizer 10 of Prior Art FIG. 1A comprises parallel walls 13, but the ultimate discharge outlet 20B is horizontal and the discharge area is less than the projected area of the screen 18 upon the discharge plane DP1. While the flow recirculation is smaller in this design, the change in direction means that the solid additives, for example pulp fibers, contained within the discretizer 10 experience significant direction changes. Given the high difference in density between pulp and air, this results in a non-homogenous mixture of air and pulp upon exiting the discretizer's ultimate discharge outlet 20B.
As shown in Prior Art FIG. 1A, the discretizer 10 exhibits a discharge area in a discharge plane DP1 oriented at a discharge angle θ of 20° or less.
Prior Art FIG. 1B illustrates another example of a discretizer, which is commercially available from Oerlikon Neumag. As shown in FIG. 1B, the discretizer 10 comprises a housing 12 that contains a rotor 14 having a plurality of swinging hammers 16. As the rotor 14 rotates in the direction of the arrow (counter-clockwise), a web of pulp 19 is introduced into the discretizer 10 such that the swinging hammers 16 individualize pulp fibers from the web of pulp 19. The individualized pulp fibers may then travel through the screen 18. The rotation of the rotor 14 imparts a strong asymmetry to the air coming out of the discretizer's discharge outlet 20 as shown in the general air flow arrows and shown in the air flow map 22. As shown by the air flow map 22, a large, but not complete portion of the air flowing out of the discretizer's discharge outlet 20 is flowing in the general direction of A. However, there is a component of the air flow that creates one or more zones of separated flow B. These one or more zones of separated flow B results in the air flow map 22 having a component that has a directional flow into the discretizer 10 not out of the discretizer's discharge outlet 20. This discretizer 10 comprises parallel walls 13 that are ostensibly tangent to the rotation of the rotor 14 and a discharge outlet 20, which is a vertical discharge outlet, from the discretizer 10 whose discharge area is greater than the projected area of the screen 18 upon the discharge plane DP1. At air flows common to use in this discretizer 10 the counter-clockwise rotation of the rotor 14 sets up an air stream, with respect to the discretizer 10 as specifically shown in FIG. 1B, that is much faster on the right side as represented by arrow A, with flow returning upwards on the left side creating one or more zones of separated flow B. This discretizer 10 exhibits a discharge area in a discharge plane DP1 oriented at a discharge angle θ of about 90°.
As the air departs the area immediately around the discretizer's rotor 14 it has a velocity vector that is strongly influenced by the counter-clockwise rotation of the rotor 14. Even if the discretizer 10 contains a screen 18, the orifices in the screen 18 do not have a large enough L/D to serve as a redirecting mechanism to more radially align the air passing through them. Subsequent to the screen 18 there are walls 13 that are parallel to approximately the tangent points of the discretizer's rotor 14. Since, by definition, the rotor 14 must rotate away from at least one of these walls 13, a flow separation occurs creating a zone of separated flow B. This flow separation, if allowed unfettered propagation, will increase to fully recirculating regions below the rotor 14 (for example between the rotor 14 and the discharge outlet 20). With solid additives, such as pulp fibers, in the air, these flow recirculations can then build up solid additives in them until gravity or other forces pull them into the stream of flow. This results in uneven solid additive flows in the system.
As shown in Prior Art FIG. 1C another example of a discretizer 10 that is similar to the discretizer 10 shown in Prior Art FIG. 1B is commercially available from Oerlikon Neumag, for example under the trade name Oerlikon Neumag 950. Like the discretizer 10 shown in FIG. 1B, this discretizer 10 comprises a housing 12 that contains a rotor 14 having a plurality of swinging hammers 16. As the rotor 14 rotates in the direction of the arrow, a web of pulp 19 is introduced into the discretizer 10 such that the swinging hammers 16 individualize pulp fibers from the web of pulp 19. The individualized pulp fibers may then travel through the screen 18. The rotation of the rotor 14 imparts a strong asymmetry to the air coming out of the discretizer's discharge outlet 20 as shown in the general air flow arrows and shown in the air flow map 22. As shown by the air flow map 22, a large, but not complete portion of the air flowing out of the discretizer's discharge outlet 20 is flowing in the general direction of A. However, there is a component of the air flow that creates one or more zones of separated flow B. These one or more zones of separated flow B results in the air flow map 22 having a component that has a directional flow into the discretizer 10 not out of the discretizer's discharge outlet 20. This discretizer 10 exhibits parallel walls 13 that are ostensibly tangent to the rotation of the rotor 14 and a discharge outlet 20, a vertical discharge outlet, from the discretizer 10 whose discharge area is greater than the projected area of the screen 18 upon the discharge plane DP1. At air flows common to use in this discretizer 10 the counter-clockwise rotation of the rotor 14 will set up an air stream, with respect to the discretizer 10 as specifically shown in FIG. 1C, that is much faster on the right side as represented by arrow A, with flow returning upwards on the left side creating one or more zones of separated flow B. This discretizer 10 exhibits a discharge area in a discharge plane DP1 oriented at a discharge angle θ of about 90°.
As the air departs the area immediately around the discretizer's rotor 14 it has a velocity vector that is strongly influenced by the rotation of the rotor 14. Even if the discretizer 10 contains a screen 18, the orifices in the screen 18 do not have a large enough L/D to serve as a redirecting mechanism to more radially align the air passing through them. Subsequent to the screen 18 there are walls 13 that are parallel to approximately the tangent points of the discretizer's rotor 14. Since, by definition, the rotor 14 must rotate away from at least one of these walls 13, a flow separation occurs creating a zone of separated flow B. This flow separation, if allowed unfettered propagation, will increase to fully recirculating regions below the rotor 14. With solid additives in the air, these flow recirculations can then build up solid additives in them until gravity or other forces pull them into the stream of flow. This results in uneven solid additive flows in the system.
One problem with existing discretizers is that they impart asymmetry to their solid additive laden air flows and/or create zones of separated flow during operation. Such asymmetrical air flows can create issues in managing the solid additives and the flows in further processes, such as fibrous structure making processes that utilize such discretizers to supply solid additives, such as pulp fibers, to the processes.
Accordingly, there is a need for a discretizer that eliminates and/or mitigates the negatives with existing discretizers; namely, eliminates and/or mitigates the asymmetry of flows exiting the discretizer and/or eliminates and/or mitigates zones of separated flow within the discretizer and a need for a method for making a fibrous structure that utilizes a discretizer that eliminates and/or mitigates the negatives associated with existing discretizers.